1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device implementing in-plane switching (IPS) where an electric field to be applied to liquid crystal molecules is generated in a plane parallel to a substrate.
2. Discussion of the Related Art
A liquid crystal display device uses the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. Liquid crystal molecules have a definite orientational alignment as a result of their long, thin shapes. That orientational alignment can be controlled by an applied electric field. In other words, as an applied electric field changes, so does the alignment of the liquid crystal molecules. Due to the optical anisotropy, the refraction of incident light depends on the orientational alignment of the liquid crystal molecules. Thus, by properly controlling an applied electric field a desired light image can be produced.
While various types of liquid crystal display devices are known, active matrix LCDs having thin film transistors and pixel electrodes arranged in a matrix are probably the most common. This is because such active matrix LCDs can produce high quality images at reasonable cost.
Recently, liquid crystal display (LCD) devices with light, thin, and low power consumption characteristics are used in office automation equipment and video units and the like. Driving methods for such LCDs typically include a twisted nematic (TN) mode and a super twisted nematic (STN) mode. Although TN-LCDs and STN-LCDs have been put to practical use, they have a drawback in that they have a very narrow viewing angle. In order to solve the problem of narrow viewing angle, in-plane switching liquid crystal display (IPS-LCD) devices have been proposed. The IPS-LCD devices typically include a lower substrate where a pixel electrode and a common electrode are disposed, an upper substrate having no electrode, and liquid crystals interposed between the upper and lower substrates.
A detailed explanation about operation modes of a typical IPS-LCD panel will be provided referring to FIGS. 1 to 3.
As shown in FIG. 1, upper and lower substrates 1 and 2 are spaced apart from each other, and a liquid crystal layer 3 is interposed therebetween. The upper and lower substrates 1 and 2 are called color filter substrate and array substrate, respectively. Pixel and common electrodes 4 and 5 are disposed on the lower substrate 2. The pixel and common electrodes 4 and 5 are parallel with and spaced apart from each other. The pixel and common electrodes 4 and 5 apply a horizontal electric field 6 to the liquid crystal layer 3. The liquid crystal layer 3 has a negative or positive dielectric anisotropy, and thus it is aligned parallel with or perpendicular to the horizontal electric field 6, respectively.
FIGS. 2A and 2B conceptually illustrate operation modes of a conventional IPS-LCD device. When there is no electric field between the pixel and common electrodes 4 and 5, as shown in FIG. 2A, the long axes of the liquid crystal molecules maintain an angle from a line perpendicular to the parallel pixel and common electrodes 4 and 5. Herein, the angle is 45 degrees, for example.
On the contrary, when there is an electric field between the pixel and common electrodes 4 and 5, as shown FIG. 2B, there is an in-plane horizontal electric field 6 parallel with the surface of the lower substrate 2 between the pixel and common electrodes 4 and 5. The in-plane horizontal electric field 6 is parallel with the surface of the lower substrate 2 because the pixel and common electrodes 4 and 5 are formed on the lower substrate 2. Accordingly, the liquid crystal molecules are twisted so as to align, for example, the long axes thereof with the direction of the horizontal electric field 6, thereby the liquid crystal molecules are aligned such that the long axes thereof are parallel with the line perpendicular to the pixel and common electrodes 4 and 5.
By the above-mentioned operation modes and with additional parts such as polarizers and alignment layers, the IPS-LCD device displays images. The IPS-LCD device has wide viewing angles since the pixel and common electrodes are together placed on the lower substrate. Moreover, the fabricating processes of this IPS-LCD device are simpler than those of other various LCD devices.
However in the IPS-LCD device, a color-shift which depends on the viewing angle still remains. It is already known that this color-shift cannot be acceptable for full color-image display. This color-shift is related to a rotational direction of the liquid crystal molecules under application of electric field when the applied voltage is greater than the threshold voltage. Moreover, this color-shift is caused by increasing or decreasing of an optical retardation (Δn·d) of the liquid crystal layer with viewing angle.
For the sake of discussing the above-mentioned problem of the IPS-LCD device, with reference to FIG. 3, the specific pixel structure of the IPS-LCD device is employed and will be described in detail.
As shown in FIG. 3, the pixel and common electrodes 7 and 8 have bend angle α. These bend electrode's structure allows the liquid crystal molecules 9 to rotate in opposite direction in each pixel when the voltage is supplied to the bend electrodes. Therefore, the bend electrodes 7 and 8 and the oppositely directed liquid crystal molecules 9 divide the pixel into two different regions with different viewing angle characteristics. And thus, the color-shift can be effectively compensated by this multi domain structure.
However, when the voltage is turned ON, extraordinary domains appear around the bottom edges of driving electrodes. These extraordinary domains degrade the picture quality and reliability of the IPS-LCD device having the bend electrodes. Namely, disclination appears at the edges of the pixel areas, and thus this disclination manifests as positional non-uniformities in the transmittance of light.
FIGS. 4A and 4B are enlarged partial plan views of pixel and common electrodes. These figures illustrate arrangement of the liquid crystal molecules and the electric field when the voltage is turned ON. As shown, a common electrode 11 is extended from a common line 23, and a pixel electrode 21 is disposed parallel with the common electrode 11. The common electrode 11 forms an acute angle with the common line 23 as depicted in a portion “A” of FIG. 4A while the pixel electrode 21 forms an obtuse angle with the common line 23 as shown in a portion “D” of FIG. 4A. When the voltage is supplied to the common and pixel electrodes 11 and 21, the electric field occurs between the common and pixel electrodes 11 and 21. However at this time, a distortion of the electric field appears around the acute and obtuse angels, the portions “A” and “D”. Thereupon, reverse rotational deformation is caused by this distortion of the electric field around the portions “A” and “D”.
Referring to FIG. 4B, when the voltage is applied to the pair of electrodes 11 and 21, the liquid crystal molecule 41 located in the parallel electric field area turns clockwise while the liquid crystal molecule 51 located in the distorted electric field area turns counterclockwise. So the orientation direction of the liquid crystal is different between the parallel electric field area and the distorted electric field area, and thus the disclination occurs in the distorted electric field area. This disclination causes a decrease in the aperture ratio, and a change of the orientation direction causes traces of the extraordinary domains. These features also affect response characteristic of the liquid crystal layer, and an afterimage phenomenon occurs in the display area